Pompeii is definitely a must-see! When I was there, they sold a little book
called "Pompeii-Herculaneum Past and Present". It has descriptions and
pictures of the ruins, with plastic overlays for many of them. The overlay shows
how archaeologists believe the site originally looked. It helps to
visualize the original look.

There is also a Past and Present book for Rome. It's even more helpful
since many of Rome's ruins are more ruined than Pompeii.